DIVING BEHAVIOR OF THE SHAG PHALACROCORAX-ARISTOTELIS (AVES, PELECANIFORMES) IN RELATION TO WATER DEPTH AND PREY SIZE

The durations of more than 4000 dives and recovery periods on the surf ace of 31 shags, Phalacrocorax aristotelis, were recorded using radio- telemetry, during three breeding seasons. Data were also collected on the depth of water where the birds were diving and the size of lesser sandeels, Ammodytes marinus, fed to young each year. On average, shags foraged in water 30 m deep; mean dive and recovery times were 62.0 +/ - 1.92 sec and 84.4 +/- 5.02 sec, respectively. These times were much longer than those recorded in previous studies and also longer than th ose predicted from allometric relationships for dive and recovery time s derived for cormorants. A large proportion (> 50%) of dives appeared to involve anaerobic metabolism. Dive and recovery times were examine d in relation to water depth, time of day and position within the divi ng sequence (dive number). In each year, dive time was positively corr elated with water depth; in 1987 dive number also had a negative effec t. The relationship between recovery time and dive time was positively accelerated, suggesting that birds were diving anaerobically on longe r dives. The percentage of the dive cycle spent underwater therefore d ecreased significantly as dive time and water depth increased. Average vertical rates of ascent and descent of shags were calculated to be b etween 1.46-1.87 m sec-1. Estimated prey capture rates were significan tly higher in 1989 (average 6.8 fish dive-1) when adults were feeding their young on small, low energy value sandeels than the two previous years (averages 1.4 and 1.9 fish dive-1, respectively) when birds were feeding on larger, higher quality fish. We calculated that, on averag e, birds spent 8-10 sec (36-48%) longer on the bottom at any given dep th in 1989, which suggested that the higher prey capture rates were as sociated with an increase in duration of the bottom phase of the dive.